EP0298098B1 - Modulateur/commutateur de micro-ondes pilote par de la lumiere ultra-rapide et faisant appel a un effet photoreactif - Google Patents

Modulateur/commutateur de micro-ondes pilote par de la lumiere ultra-rapide et faisant appel a un effet photoreactif Download PDF

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Publication number
EP0298098B1
EP0298098B1 EP88900731A EP88900731A EP0298098B1 EP 0298098 B1 EP0298098 B1 EP 0298098B1 EP 88900731 A EP88900731 A EP 88900731A EP 88900731 A EP88900731 A EP 88900731A EP 0298098 B1 EP0298098 B1 EP 0298098B1
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Prior art keywords
diode junction
microwave switch
switch according
microwave
region
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Expired - Lifetime
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EP88900731A
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German (de)
English (en)
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EP0298098A1 (fr
Inventor
Michael De La Chapelle
Hui-Pin Hsu
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Raytheon Co
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Hughes Aircraft Co
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/80—Optical aspects relating to the use of optical transmission for specific applications, not provided for in groups H04B10/03 - H04B10/70, e.g. optical power feeding or optical transmission through water
    • H04B10/806—Arrangements for feeding power
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01Q—ANTENNAS, i.e. RADIO AERIALS
    • H01Q3/00—Arrangements for changing or varying the orientation or the shape of the directional pattern of the waves radiated from an antenna or antenna system
    • H01Q3/26—Arrangements for changing or varying the orientation or the shape of the directional pattern of the waves radiated from an antenna or antenna system varying the relative phase or relative amplitude of energisation between two or more active radiating elements; varying the distribution of energy across a radiating aperture
    • H01Q3/2676—Optically controlled phased array

Definitions

  • This invention relates to a microwave switch comprising:
  • a microwave switch of this kind but for the example of an avalanche diode, is disclosed in R.A. Kiehl, "An Avalanching Optoelectronic Microwave Switch", IEEE Transactions on Microwave Theory and Techniques, Volume MTT-27, No. 5, May 1979, pp. 533 - 539.
  • the invention also relates to a corresponding microwave modulator, and to a method for altering, in particular switching or modulating, electromagnetic signals.
  • the invention relates to a microwave switch/ modulator which is controlled by optical illumination from a laser light source or the like.
  • the illumination causes variations in the reactance of the switch/ modulator photodiode, thereby detuning a resonant circuit, causing RF signals to be reflected rather than absorbed or transmitted.
  • High-speed RF and microwave switches are used in numerous applications. They are often used in radar receivers as blanking switches to protect sensitive circuitry from damage during the transmit pulse. Fast switching is necessary for this application in order to minimize the period of time in which the receiver is disabled after the transmit pulse has ended so the radar can detect close targets.
  • Plasma switches work by forming a highly conductive plasma in the electrical path of a microwave transmission line.
  • the plasma-forming semiconductor material can be placed as a shunt element, a series element or both. See Platte "Optoelectronic Microwave Switching", IEEE Proceedings, Volume 132, No. 2, pp. 126-132, April 1985.
  • Plasma switches turn on very rapidly (10-100 ps) but turn off very slowly (1 ⁇ s-1 ms) because there is no electric field present to sweep away carriers.
  • the turnoff time is determined by carrier recombination unless some means of shunting is used as described above.
  • a second disadvantage of the plasma switch is the requirement for a high-powered laser to form a plasma over a large region or surface.
  • Avalanche photodiode switches have been used as microwave switches because of their high photoconductivity.
  • the diode is typically biased near avalanche breakdown, where a small amount of illumination causes ionization in the high field region.
  • the electron-hole pairs in turn ionize other atoms creating large photocurrents through multiplication.
  • Avalanche photodiodes switch comparatively slow because the avalanche process takes time to build. Also, they are noisy and can have thermal problems.
  • Bias controlled optoelectronic switches represent a whole class of switches which use bias control to turn optical detectors on and off. For instance, a photodiode can switch on and off the detected microwave modulated illumination from a laser by transitioning from reverse to forward bias, respectively. In other words, photodiode detectors only function in reverse bias. When they are switched to forward bias, they will not detect amplitude modulated microwave energy on an optical carrier. In a similar way, an avalanche photodiode can be turned on and off by switching the bias in and out of avalanche. Bias controlled optoelectronic switches thus rely on changes in bias in order to switch. As a result, this type of switch has the same speed as a conventional PIN diode switch (on the order of 10 ns).
  • a gold-type GaAs surface barrier diode is described in T. Yamamoto and Y. Ota, "The Gold n -Type GaAs Surface Barrier Diode and its Application to Photocapacitors", Solid State Electronics, Vol. 11, No. 2 (1968.2), pp. 219-224.
  • the gold-coated diode discussed in this document is sensitive to illumination and changes its capacitance in dependence thereof. It is used in oscillator - however, without being biased -, and changes the oscillator's frequency.
  • a variable voltage is provided to reverse-bias the diode junction to thereby control the operating frequency of the microwave switch, mentioned at the outset.
  • the microwave switch/ modulator is electronically tuneable so that the microwave frequency of operation can be varied by adjusting the reverse bias voltage to the photodiode.
  • the microwave switch according to the present invention can switch or modulate microwave signals at a factor 10-1000 times faster than conventional bias controlled microwave switches such as PIN diodes.
  • the invention employs an optically controlled diode which uses changes in diode reactance, rather than resistance, as a mechanism to switch or modulate microwave energy.
  • the optically controlled switch of the invention also enjoys complete isolation between the control circuitry (pulse modulator for the laser) and the microwave switching circuitry, so that no undesirable interference or transients can be coupled into the switched signal.
  • the invention is thus useful in a wide range of high-speed RF and microwave switching applications, including EW receiver blanking switches for applications where a wide bandwidth tuneability can be employed for selectively receiving and nulling jamming signals.
  • the invention is also useful for analog modulator applications where an intensity modulated optical signal can be used to amplitude modulate a microwave signal.
  • the range of applications of the invention includes radar, EW, RF and microwave communications, and signal processing.
  • the present invention employs a mechanism in which the reactance of the photodiode varies with illumination.
  • This mechanism will be hereinafter referred to as the "photoreactive effect”.
  • Photoreactance is caused by the formation of a plasma on the surface of a photodiode resulting from photons colliding with atoms (ionization) in the active region of the device.
  • the photoreactive diode 20 comprises ohmic contact 22, undepleted region 24, depletion region 26, plasma region 28 and a Schottky contact ring 30.
  • the thickness of the plasma region, t p increases with illumination on the photodiode.
  • a depletion region of thickness, t d is formed in a reversed biased diode.
  • the capacitance of the photodiode can be approximated by the simple parallel plate model as: Where A is the active area of the photodiode.
  • the absorption coefficient, ⁇ is a measure of the degree to which a semiconductor absorbs photons and is, of course, a function of wavelength, as shown in FIG. 3.
  • the conductivity versus depth is plotted in FIG. 4 for two different illumination levels.
  • the exponential conductivity distribution can be approximated by a rectangular distribution as depicted by the dotted line in FIG. 4 of conductivity ⁇ p and thickness t p .
  • the effective plasma thickness is defined as a region of approximately metallic conductivity, where ⁇ p is on the order of 105 mhos/cm or greater.
  • Equation (8) shows that the capacitance increases with increasing illumination and decreasing bias voltage. The capacitance becomes very large as t p approaches t d .
  • the depletion width is not affected by the presence of the plasma, since the plasma is charge neutral -- containing as many holes as electrons.
  • the equilibrium density of carriers within the plasma region is determined by the rate at which electrons and holes are created, the rate at which they drift and diffuse, and their recombination times.
  • Electrons and holes are separated and carried by the electric field present in the depletion region. The electrons are carried across the depletion region creating an external photocurrent while the holes move in the opposite direction and are immediately collected at the Schottky contact. The space charge of the electrons travelling through the drift region will depress the electric field, but this is considered a second order effect and is neglected.
  • the measured reflection coefficients (S11) as a function of frequency for an unilluminated 50 ⁇ m diameter, GaAs, Schottky photodiode are plotted on the Smith chart of FIG. 5A.
  • the S-parameter data is fitted to the model of FIG. 5B where C d is the depletion capacitance, R d is the contact resistance and L b is the bond wire inductance.
  • Illuminating the active area of the photodiode with 2mW of optical energy from a miniature GaAlAs laser diode causes the reflection coefficient at 8 GHz to slightly shift, as shown in FIG. 6.
  • the change in the S11 is purely reactive and corresponds to an increase in the depletion capacitance from 0.52 pF to 0.59 pF with illumination for 8 volts bias. With 2 volts bias the capacitance changes from 1.47 pF to 1.81 pF with illumination. There is a larger change in junction capacitance at 2 volts bias than at 8 volts bias (23% versus 7.3%) This is due to the fact that the depletion width is much narrower at 2 volts bias than at 8 volts bias.
  • a simple microwave matching circuit was designed and fabricated which matches 50 Ohms to the impedance of the unilluminated diode at 8 GHz.
  • the matching circuit consisted of a series inductor and a quarter wavelength impedance transformer, as shown in FIG. 7a.
  • the series inductor is chosen to resonate the unilluminated capacitance of the photodiode, C du , at 8 GHz.
  • L m eliminates the reactie part of the photodiode impedance leaving the real part, R d .
  • a transformer of such low impedance can be conveniently realized on microstrip using a thin substrate of high dielectric constant. For these reasons, 0,254 mm (10 mil) thick aluminum is chosen.
  • the series matching inductor is realized on microstrip as a short section of high impedance transmission line. A layout of the circuit is shown in FIG. 7b.
  • Other two-element matching circuits are also suitable for this application. These matching circuits may be realized on other microwave transmission media such as coaxial line, waveguide and strip-line.
  • the impedance matching circuit is designed to give a high return loss of its input over a narrow band about the center frequency (8 GHz).
  • the center frequency at which the return loss peaks can be varied by adjusting the reverse bias to the photodiode.
  • the tuning arises from the variation of the depletion capacitance with bias voltage as given by equation (9).
  • a plot of return loss versus frequency for several reverse bias voltages is shown in FIG. 8a.
  • the frequency at which the return loss peak occurs as a function of bias voltage is plotted in FIG. 8b. Notice that the frequency levels off at higher bias voltages as the diode punches through.
  • Illuminating the diode also causes the junction capacitance to vary, as described above.
  • FIG. 9 shows the return loss versus frequency for an illuminated (using a GaAlAs laser of 0.833 ⁇ m wavelength) and unilluminated photodiode with 8 volts bias.
  • the resonant frequency shifts by 300 MHz corresponding in a change in C d from 0.55 pF in the unilluminated state to 0.59 pF in the illuminated state.
  • the increase in capacitance is attributed to the formation of a plasma region in the diode, as described above.
  • the wavelength of the GaAlAs laser optical control signal (0.833 ⁇ m) is slightly shorter than the critical wavelength of the GaAs photodiode (0.900um), photons are absorbed in the photodiode to create a plasma, as described by equation (4).
  • FIG. 9 shows that the return loss at 8 GHz can be made to vary from 25 dB to 3 dB simply by illuminating the photodiode with 2mW of optical power. The light is directed to the active area of the photodiode by an optical fiber.
  • the circuit can be made into a useful switch by connecting a circulator on the input (see FIG. 10a).
  • a circulator on the input
  • the return loss off of the circuit is high, the incident energy is absorbed and the switch is in the high isolation state.
  • the return loss is low, incident energy is reflected and the device is in the low insertion loss state.
  • the performance of the switch at two diferent bias voltages is shown in FIG. 10b. Notice that the frequency shift due to illumination is much greater for smaller reverse bias voltages, as explained by equation (8).
  • the center frequency of operation can be electronically tuned over a 25% band width by varying the reverse bias voltage.
  • the invention has demonstrated operation at X-band, although it is theoretically possible to design a photoreactive switch/modulator which functions at MMW frequencies.
  • This invention switches states on the order of 10 pS, which is the time it takes to form the conducting plasma.
  • the turn off time is determined by the time it takes for the plasma to recombine, diffuse, or be swept away by the electric field.
  • the advantage of this switch is that the plasma is formed in a high electric field region so that most of the carriers are rapidly swept away when the illumination is stopped.
  • Other plasma switches operated by photoconductive described in technical journals form plasmas in bulk semiconductors with no electric field to sweep away carriers.
  • the turn off time for other plasma switches is determined by the recombination time of the carriers.
  • This invention can use a low power laser to generate the optical control pulses or modulation.
  • a laser is a threshold device -- when the bias current exceeds some threshold value it begins to lase almost instantaneously.
  • the Ortel Corporation laser may be used to switch the invention off and on at picosecond speeds. Test equipment does not exist which permits the measurement of such fast picosecond pulses. Instead, we determined the switching speed of the device by modulating the laser control signal with a sine wave at X-band frequencies. The photodiode responded to X-band optical control signals indicating a switching speed on the order of 10pS, which is a factor of 1000 improvement over a conventional PIN diode switch.
  • two-port transmission/reflection switch using a photoreactive diode rather than the one-port reflection/absorption switch described above.
  • the former does not require a circulator to separate the input and output signals and can be used to switch higher power RF energy.
  • two-port switches could be fabricated as shown in FIG. 11. The two schemes function by detuning a high Q resonant circuit when the photodiode is illuminated.
  • the resonant circuit is formed by the unilluminated photodiode capacitance, C du , and a resonant circuit inductance, L R .
  • L R is chosen according to, where ⁇ o is the operating frequency of the switch.
  • Both switches reflect incident microwave signals at frequency ⁇ o when the photodiode is unilluminated. When illuminated, the resonant circuit is detuned allowing the signal to be transmitted.
  • the design procedure and functioning of the switch is basically the same as described above.
  • GaAs gallium arsenide
  • the invention may be practiced with other semiconductor materials as well.
  • the semiconductor material is chosen to suit the particular wavelength illumination. Suitable materials include: silicon (Si), germanium arsenide (GeAs) and gallium arsenide (GaAs) for 0.8 ⁇ m illumination; indium gallium arsenide (In GaAs) and indium arsenide phosphate (In GaAsP) for 1.3 ⁇ m illumination; and indium gallium arsenide (InGaAs) for 1.55 ⁇ m illumination.
  • the present invention provides a microwave switch/modulator which employs the photoreactive mechanism to switch at ultra-high speed.
  • the invention is applicable to a wide range of frequencies including, microwave, RF, and MMW frequencies.

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Variable-Direction Aerials And Aerial Arrays (AREA)
  • Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)
  • Semiconductor Lasers (AREA)
  • Light Receiving Elements (AREA)

Abstract

Le commutateur/modulateur de HF/micro-ondes utilise une diode (20) à commande optique. On peut faire varier la réactance de la diode en faisant varier l'intensité d'éclairage. De cette manière, la photodiode, conjointement avec un circuit externe, peut commuter ou moduler un signal de micro-ondes en faisant varier la réactance de la diode au moyen d'une source de lumière laser ou analogue. On peut faire varier la tension de polarisation pour syntoniser électroniquement la diode de manière à permettre la commande électronique de la fréquence micro-ondes de fonctionnement.

Claims (22)

  1. Elément de commutation micro-onde comprenant:
    - un matériau semiconducteur (24, 26) définissant une jonction de diode;
    - une région photoactive sur le matériau semiconducteur (26), cette région photoactive ayant une réactance qui varie en fonction de l'illumination optique;
    - une source d'illumination optique dirigée vers la région photoactive pour former une région de plasma (28) adjacente à la région photoactive;
    caractérisé par une source de tension variable pour polariser en inverse la jonction de diode, pour commander ainsi la fréquence de fonctionnement de l'élément de commutation micro-onde.
  2. Elément de commutation micro-onde selon la revendication 1, caractérisé en ce que la source de tension variable établit une région désertée (26) et une région non désertée (24).
  3. Elément de commutation micro-onde selon la revendication 1 ou 2, dans lequel la profondeur de la région de plasma (28) est variable en fonction de l'énergie de l'illumination optique que reçoit la région photoactive.
  4. Elément de commutation micro-onde selon la revendication 3, caractérisé en ce que la réactance de la jonction de diode est variable avec la profondeur de la région de plasma (28).
  5. Elément de commutation micro-onde selon l'une quelconque des revendications précédentes, caractérisé en ce que la source d'illumination optique est une source commandée.
  6. Elément de commutation micro-onde selon l'une quelconque des revendications précédentes, caractérisé par :
    - des moyens pour insérer la jonction de diode dans un chemin de signal électromagnétique, ce signal électromagnétique pouvant être commuté par la commande de la source d'illumination optique; et
    - un circuit résonnant couplé à la jonction de diode pour être désaccordé sélectivement lorsque la région photoactive est illuminée.
  7. Elément de commutation micro-onde selon la revendication 6, caractérisé en ce que le circuit résonnant est un circuit d'adaptation d'impédance (ZM, Lm) qui est couplé à la jonction de diode pour annuler et faire résonner pratiquement la réactance de la jonction de diode lorsque la région photoactive n'est pas illuminée.
  8. Elément de commutation micro-onde selon la revendication 7, caractérisé en ce que le circuit d'adaptation d'impédance (ZM, Lm) transforme la réactance de la jonction de diode en une impédance de système d'une valeur nominale de 50 ohms.
  9. Elément de commutation micro-onde selon la revendication 7 ou 8, caractérisé en ce que le circuit d'adaptation d'impédance (ZM, Lm) et la jonction de diode définissent un élément de commutation à réflexion/absorption à un seul accès.
  10. Elément de commutation micro-onde selon l'une quelconque des revendications précédentes, caractérisé en ce que le matériau semiconducteur (24, 26) comprend au moins un contact ohmique (22).
  11. Elément de commutation micro-onde selon l'une quelconque des revendications précédentes, caractérisé en ce que le matériau semiconducteur (24, 26) comprend au moins un contact Schottky.
  12. Elément de commutation micro-onde selon l'une quelconque des revendications précédentes, caractérisé en ce que le matériau semiconducteur (24, 26) est sélectionné dans le groupe qui comprend le silicium, l'arséniure de gallium, l'arséniure de germanium, l'arséniure d'indium-gallium et l'arséniure-phosphure d'indium-gallium.
  13. Elément de commutation micro-onde selon l'une quelconque des revendications précédentes, caractérisé en ce que la source d'illumination optique est un laser.
  14. Elément de commutation micro-onde selon l'une quelconque des revendications précédentes, caractérisé par un circulateur qui est destiné à établir des accès d'entrée et de sortie séparés pour la jonction de diode.
  15. Elément de commutation micro-onde selon l'une quelconque des revendications précédentes, caractérisé par une inductance résonnante (LR) couplée à la jonction de diode pour définir un élément de commutation à transmission/réflexion à deux accès.
  16. Modulateur micro-onde, caractérisé en ce qu'il comprend un élément de commutation micro-onde selon l'une au moins des revendications 6 à 15, et dans lequel la modulation de la source d'illumination optique a pour effet de moduler en amplitude un signal micro-onde qui est appliqué à la jonction de diode.
  17. Procédé pour modifier, en particulier commuter ou moduler, des signaux électromagnétiques, comprenant les étapes suivantes :
    - on applique les signaux électromagnétiques à un matériau photoréactif constitué par un matériau semiconducteur (24, 26) définissant une jonction de diode et une région photoactive sur le matériau semiconducteur (26), cette région photoactive ayant une réactance qui varie avec l'illumination optique;
    - on illumine la région photoactive avec une source d'illumination optique pour former une région de plasma (28) adjacente à la région photoactive;
    caractérisé par l'étape suivante :
    - on applique un signal de source de tension variable à la jonction de diode, pour polariser en inverse cette jonction de diode, et pour commander ainsi la fréquence de fonctionnement des signaux électromagnétiques.
  18. Procédé selon la revendication 17, caractérisé par l'étape suivante :
    - on commande la source d'illumination optique.
  19. Procédé selon la revendication 17 ou 18, caractérisé par l'étape suivante :
    - on couple un circuit résonnant à la jonction de diode pour désaccorder sélectivement cette jonction de diode lorsque la région photoactive est illuminée.
  20. Procédé selon l'une quelconque des revendications 17 à 19, caractérisé par l'étape suivante :
    - on sélectionne le matériau semiconducteur (24, 26) dans le groupe qui comprend le silicium, l'arséniure de gallium, l'arséniure de germanium, l'arséniure d'indium-gallium et l'arséniure-phosphure d'indium-gallium.
  21. Procédé selon l'une quelconque des revendications 17 à 20, caractérisé par l'étape suivante :
    - on connecte un circulateur à la jonction de diode, dans le but d'établir des accès d'entrée et de sortie séparés pour la jonction de diode.
  22. Procédé selon l'une quelconque des revendications 17 à 21, caractérisé par l'étape suivante :
    - on couple une inductance résonnante (LR) à la jonction de diode pour définir un élément de commutation à transmission/réflexion à deux accès.
EP88900731A 1986-12-31 1988-07-19 Modulateur/commutateur de micro-ondes pilote par de la lumiere ultra-rapide et faisant appel a un effet photoreactif Expired - Lifetime EP0298098B1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US06/948,339 US4832433A (en) 1986-12-31 1986-12-31 Fiber-optic feed network using series/parallel connected light emitting opto-electronic components
US948339 1986-12-31

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EP0298098A1 EP0298098A1 (fr) 1989-01-11
EP0298098B1 true EP0298098B1 (fr) 1994-03-02

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EP88900730A Expired - Lifetime EP0294465B1 (fr) 1986-12-31 1987-12-04 Reseau d'alimentation par fibres optiques utilisant des composants opto-electroniques electroluminescents branches en serie ou en parallele
EP88900731A Expired - Lifetime EP0298098B1 (fr) 1986-12-31 1988-07-19 Modulateur/commutateur de micro-ondes pilote par de la lumiere ultra-rapide et faisant appel a un effet photoreactif

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US (1) US4832433A (fr)
EP (2) EP0294465B1 (fr)
JP (1) JPH01502471A (fr)
DE (2) DE3777407D1 (fr)
IL (1) IL84686A (fr)
WO (1) WO1988005215A1 (fr)

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WO2005103798A2 (fr) * 2004-04-14 2005-11-03 The Government Of The United States Of America, As Represented By The Secretary Of The Navy Amplificateur d'hyperfrequence photonique
DE102004024367A1 (de) 2004-05-17 2005-12-22 Rohde & Schwarz Gmbh & Co. Kg Kalibrierbare Mikrowellen-Schaltung mit beleuchtbaren GaAs-FET sowie Kalibriervorrichtung und Verfahren zur Kalibrierung
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JPS5928098B2 (ja) * 1981-03-17 1984-07-10 日本電信電話株式会社 光ケ−ブル伝送方式
JPS59204329A (ja) * 1983-05-04 1984-11-19 Sumitomo Electric Ind Ltd 多方向通信用光送信器
US4546249A (en) * 1983-07-01 1985-10-08 The United States Of America As Represented By The Secretary Of The Navy High speed optically controlled sampling system
DD219923A1 (de) * 1983-12-08 1985-03-13 Inst Elektro Anlagen Elektro-optische schaltungsanordnung zur verbindung von signalverarbeitenden einrichtungen
GB2165712B (en) * 1984-10-17 1988-05-11 Stc Plc Power transmission
IT1184245B (it) * 1985-06-20 1987-10-22 Pirelli Cavi Spa Apparecchiatura di telealimentazione a guida ottica

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE102005021298A1 (de) * 2005-05-09 2006-11-16 Rohde & Schwarz Gmbh & Co. Kg Optoelektronisch gesteuerter Schalter oder Modulator und Abschwächer
DE102007041829A1 (de) 2007-09-03 2009-03-05 Siemens Ag Elektronenquelle
DE102007041829B4 (de) * 2007-09-03 2009-08-20 Siemens Ag Elektronenquelle
US7787595B2 (en) * 2007-09-03 2010-08-31 Siemens Aktiengesellschaft Electron source

Also Published As

Publication number Publication date
DE3777407D1 (de) 1992-04-16
EP0298098A1 (fr) 1989-01-11
EP0294465B1 (fr) 1992-03-11
DE3789229D1 (de) 1994-04-07
EP0294465A1 (fr) 1988-12-14
US4832433A (en) 1989-05-23
WO1988005215A1 (fr) 1988-07-14
DE3789229T2 (de) 1994-10-06
IL84686A (en) 1993-02-21
JPH01502471A (ja) 1989-08-24

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